cycle (PWM style?) and — if possible
— is random between the three
lights?

— Mark Bender

AFor randomness, you would need a microcontroller and that could also do the PWM. I suggest you
change the lights from 110 VAC to
12 VDC; 12V bulbs are available
from a camping store. Now, I see that
a PIC PWM command does not
accept a variable, so that makes it
difficult to produce variable PWM.
Consequently, I will use three
separate sweeps at different
frequencies to have a random effect.

The circuit in Figure 6 is one of
three needed for the three lamps.
IC1 is the clock — about 100 Hz —
and could be common to all three
circuits if you want. R1 is included to
ensure that the period of IC1
oscillation is longer than that of IC2.
The 555 can’t be re-triggered while it

■ FIGURE 6.

is timing, so if the period of IC1 is
less or equal to the period of IC2,
IC2 will not be triggered every time.
If the lights suddenly dim during the
sweep, R1 needs to be larger.

IC2 is PWM’d by a triangle wave
generated by IC3. IC3A is a hysteresis
oscillator swinging between 1/3 VCC
and 2/3 VCC. The frequency of the
triangle wave is determined by R5,